In the manufacture of steel products, a coating is often applied to the finished product. The main objective is to protect the steel product against corrosion. Coatings containing zinc or zinc alloys cover by far the largest share of the market for this purpose. However, tin and chrome coatings also have significant share in the packing plate sector of the market.
In continuous processes for coating a steel strip, two processes in particular have attained major importance, namely, hot galvanizing and electrolytic galvanizing or electrolytic coating. Hot galvanizing, also referred to as the hot-dip process, immerses a steel substrate in a molten zinc bath where the zinc forms an alloy with the iron on the surface of the steel substrate. Electrolytic galvanizing places the steel substrate in an electrolyte bath containing zinc ions and applies an electric current to the bath to plate the zinc onto the steel substrate.
In both cases, a cold-rolled strip is the initial material used. The cold-rolled strip is formed from a hot-rolled strip having an initial thickness greater than the thickness of the desired product. Typically, the hot-rolled steel strip has a thickness of 1.5 to 5 mm. The hot-rolled steel strip is then cold-rolled to a final thickness of 0.3 to 2 mm.
Before the hot-rolled strip can be cold-rolled, the scale that has formed during hot-rolling must be removed. The scale from the hot-rolling process is a surface layer made up of different iron oxides with a typical thickness of about 5 to 20 μm. The scale is removed by decomposing the scale layer in mineral acids, such as sulfuric, hydrochloric acid and mixtures of various acids. This process is known as pickling and is implemented in batch-type or continuous pickling plants. One configuration of a pickling apparatus for this process is described, for example, in Austrian Patent No. AT 399,517.
Various efforts have been made to combine individual process stages in the production of steel strips in order to reduce operating costs for the plants and to improve the quality of the product. The current state of the art, for example, is to integrate the pickling and cold-rolling processes in a single building so that the hot rolled teel is fed directly to the cold rolling machinery. German Patent No. DE-PS 1 960 6305 C1 assigned to Mannesmann, on the other hand, describes an apparatus for carrying out the hot-rolling and pickling processes in one plant.
The steel hardens during the cold-rolling processes for forming a steel strip. The hardening of the steel must be reversed in a subsequent annealing process before the steel can be used. This annealing process is usually conducted in a batch operation in a hood-type furnace or in a continuous process in a continuous, through-type furnace. In hot galvanizing, the annealing and coating processes are combined in a single strip treatment plant, where the annealing process takes place first and the strip is then covered with a layer of zinc in a further stage by being dipped into a molten zinc bath.
Another process of depositing zinc and other metals on a steel strip is the electrolytic process. In this case, the strip, which is usually an annealed, cold-rolled strip, is pulled through an electrolyte bath containing dissolved metal ions. The strip is connected to the negative pole of a rectifier (the cathode) by a suitable connection, while metallic anodes connected to the positive pole of the rectifier are mounted opposite the strip. By applying an electric current between these poles, the metal ions are deposited on the steel strip in metallic form.
An electrolytic process of this type is described, for example, in Austrian Patent No. AT 373,922. A different process, which is particularly suitable for depositing zinc alloys from chloride electrodes because the anode area is separated from the cathode area by a diaphragm, is described in EP 580 730 (SIKEL).
Over the past few years, the production process for producing a hot-rolled steel strip has seen significant development and it has become possible to produce a thin strip directly by a hot-rolling process that could previously be obtained only by cold-rolling processes. As a result, this hot-rolled strip can be used in many applications which were previously reserved for a cold-rolled strip. In these processes, the corrosion protection layer must be applied to the hot-rolled strip. Japanese Patent No. JP 91258210, therefore, suggests de-scaling hot-rolled strip in a dry process and then feeding the cleaned steel strip to a hot-galvanizing process. This process, however, has some disadvantages because dry de-scaling processes are expensive. In addition, there is no longer an advantage in combining the annealing and hot-galvanizing processes, which is an advantage for cold-rolled strips, because there is no need to anneal the hot-rolled strip.
Accordingly, there is a continuing need in the industry for an improved process of coating a hot-rolled steel sheet.